Benzotristhiazole based chromophores for nonlinear optics

Article abstract of DOI:10.1016/j.molstruc.2012.06.018

Methylation of unsymmetrical trimethylbenzotristhiazole with various reagents has been studied in details. The structure of obtained 2,3,5,8-tetramethylbisthiazolo[4,5-e;5,4-g]-1,3-benzothiazolium iodide has been proved by X-ray analysis in accord with the quantum-chemical calculations at DFT-B3LYP level. The electron-withdrawing tetramethylbenzotristhiazolium building block was submitted to Knoevenagel reaction with aromatic aldehydes containing an electron-releasing group. This led to a series of condensation products with push-pull structure. Relations between structure, linear (UV-Vis and fluorescence spectra) and nonlinear optical properties (calculated frontier orbital energy gap, BLA index, hyperpolarizabilities β and γ) have been investigated.

Full text of DOI:10.1016/j.molstruc.2012.06.018

Journal of Molecular Structure 1027 (2012) 70–80
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Benzotristhiazole based chromophores for nonlinear optics
a
a
b
Andrea Fülöpová a, Peter Magdolen a, , Ivica Sigmundová , Pavol Zahradník , Erik Rakovsky ,
Marek Cigán
⇑
´
ˇ c
a Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic
b Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic
c Chemical Institute, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic
h i g h l i g h t s
"
"
"
"
"
Various methylations of unsymmetrical trimethylbenzotristhiazole were studied.
The structure of the obtained monomethylated salt was proved by X-ray analysis.
The X-ray analysis was in accord with the quantum-chemical calculations (DFT-B3LYP).
A series of condensation products with push–pull structure was prepared.
Relations between structure, linear and nonlinear optical properties were examined.
a r t i c l e i n f o
a b s t r a c t
Article history:
Methylation of unsymmetrical trimethylbenzotristhiazole with various reagents has been studied in
details. The structure of obtained 2,3,5,8-tetramethylbisthiazolo[4,5-e;5,4-g]-1,3-benzothiazolium iodide
has been proved by X-ray analysis in accord with the quantum-chemical calculations at DFT-B3LYP level.
The electron-withdrawing tetramethylbenzotristhiazolium building block was submitted to Knoevenagel
reaction with aromatic aldehydes containing an electron-releasing group. This led to a series of conden-
sation products with push–pull structure. Relations between structure, linear (UV–Vis and fluorescence
spectra) and nonlinear optical properties (calculated frontier orbital energy gap, BLA index, hyperpolar-
Received 24 April 2012
Received in revised form 8 June 2012
Accepted 11 June 2012
Available online xxxx
Keywords:
Benzotristhiazole
Azolium salts
X-ray analysis
DFT calculations
Hyperpolarizability
UV–Vis spectra
izabilities b and c) have been investigated.
Ó 2012 Published by Elsevier B.V.
1. Introduction
p-excessive unit (thiophene, pyrrole, carbazole) were found as
highly efficient NLO-phores [8–11]. The heterocyclic ring also
brings photochemical and thermal stability into the molecule.
In addition to the classical push–pull dipolar molecules where a
linear charge transfer is the principal factor of optical nonlinearity,
compounds with quadrupolar, octupolar, multiple-branched, or
dendrimeric structures have been reported displaying important
cooperative effects leading to enhanced NLO-response [12,13].
Organic molecules exhibiting sufficient nonlinear optical (NLO)
response (first or second hyperpolarizabilities – b, ; two-photon
c
absorption – TPA) have obtained a great deal of attention for their
potential application as active elements in optoelectronic devices
including microscopy [1,2], optical power limiting [3], photody-
namic therapy [4], up-converted lasing [5], optical data storage
[6], and microfabrication [7]. The quantitative measures of NLO-re-
sponse are expressed by the experimental or theoretical values of
Recently, we have published
a six-step synthesis of the
unsymmetrical benzotristhiazole (BTT) unit (Scheme 4) as a novel
electron-withdrawing core for construction of possible NLO chro-
mophores [14].
Continuing our work in this line, we herein wish to report the
synthesis and study of compounds based on benzotristhiazole
building block. To increase the electron-withdrawing strength of
the heterocyclic central core, we have tried to quaternize the thia-
zole nitrogen. In addition to the improvements of the electronic
structure of the molecule, the ionic compounds are especially
hyperpolarizabilities b or
absorption is quantified by the calculated or measured TPA cross-
sections rTPA
It was shown that chromophores containing a
c, while the efficiency of two-photon
.
p-deficient
heterocyclic ring (pyridine, quinoline, benzothiazole) as well as a
⇑
Corresponding author. Tel.: +421 2 60296603; fax: +421 2 60296337.
E-mail address: magdolen@fns.uniba.sk (P. Magdolen).
0022-2860/$ - see front matter Ó 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molstruc.2012.06.018

A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
71
attractive among the wide range of NLO-phores due to their high
stability and tailorability.
((CD3)2SO, 75 MHz): d = 175.2, 171.7, 169.1, 142.1, 138.3, 133.4,
129.9, 126.7, 121.1, 48.5 (CH3 from CH3OH), 39.5, 20.1, 19.9, 17.24.
15N NMR ((CD3)2SO, 600 MHz): d = ꢂ79.7, ꢂ83.3, ꢂ184.1.
The asymmetry of the starting benzotristhiazole induces small
differences in the nucleophilicity of the heterocyclic nitrogen
atoms. These variations were also predicted by the quantum chem-
ical calculations. Methylation of the starting benzotristhiazole pro-
duced a mixture of two monomethylated isomers; in some cases
also dimethylated products were formed. The yield and the ratio
of the N-methylated products were influenced by the alkylating
agent and reaction conditions. The right structure of the prevailing
monomethylated product has been determined by X-ray analysis.
The one-armed condensation products of the N-methyl salt
have been prepared by the Knoevenagel reaction with aromatic
aldehydes containing an electron-releasing group.
2.2.2. General procedure for preparation of 3a–3f
In a glass vessel for microwave reactor the compound 2b (0.1 g,
0.23 mmol) was partly dissolved in methanol (10 mL). The appro-
priate aldehyde (0.242 mmol, 1.05 eq.) was added and the reaction
mixture was exposed to microwave irradiation. For accurate tem-
peratures, reaction times and obtained pressures see bellow in Ta-
ble 1. When the reaction was finished the dark precipitate with
metallic lustre was filtered off and washed with diethyl ether.
2.2.3. 2-{(E)-2-[4-(dimethylamino)-phenyl]vinyl}-3,5,8-trimeth
ylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazolium iodide (3a)
mp 317–320 °C (the dark color changes to orange at 270 °C).
Anal.: Calcd. for C22H21IN4S3: C, 46.81; H, 3.75; N, 9.92; S, 17.04.
Found: C, 46.72; H, 3.47; N, 9.66; S, 17.09. IR (powder film):
2. Experimental part
2.1. General remarks
m
= 3041 and 3009 (@CAH, CArAH), 2962 and 2906 and 2856 and
2820 (Csp3AH), 2686, 1729, 1611 and 1573 (C@C, CAr@CAr), 1520,
1432, 1366, 1259, 1092, 1020, 793 cmꢂ1 1H NMR ((CD3)2SO,
1,2-Dichloroethane was dried using a common method – reflux on
CaH2. For reactions performed under microwave irradiation either
the reactor Anton Paar Monowave 300 (max. power 850 W) or the
reactor Initiator™ BIOTAGE (max. power 300 W) were used. Melting
points were measured on a Kofler apparatus Electrothermal IA-9200
and were not corrected. 1H NMR and 13C NMR spectra were recorded
either on a Varian NMR System™ 300 (300 MHz) spectrometer, or in
some cases a Varian NMR System™ 600 (600 MHz) was used. 15N
chemical shifts were measured indirectly, via 1HA15N long range cor-
relation on a Varian NMR System™ 600 MHz spectrometer. The
chemical shifts were referenced to a tetramethylsilane standard, the
shifts for 15N spectra are given on the nitromethane scale. IR spectra
were recorded on Thermo Scientific Nicolet iS10 spectrometer (Smart
iTR diamond ATR). Electronic absorption spectra were obtained on a
Jenway 6705 UV/Vis spectrophotometer and fluorescence measure-
ments were performed on an Edinburgh Instruments FSP 920 spec-
trofluorimeter. The fluorescent quantum yield of derivatives 3a–3f
insolution wasdeterminedusingRhodamine 101 inEtOH asthe stan-
dard, taking the quantum yield of Rhodamine 101 in EtOH equal to
1.00 [15]. In the condensation reactions several aldehydes were used,
which are either commercially available (4-(dimethylamino)benzal-
dehyde and 4-(dimethylamino)cinnamaldehyde) or were
prepared according to the literature: (2E,4E)-5-[4-(N,N-dimethyl-
amino)phenyl]penta-2,4-dienal [16], 4-(diphenylamino)benzalde-
hyde [17], (2E)-3-[4-(N,N-diphenylamino)phenyl]propenal [18],
julolidine-9-carbaldehyde [19].
.
600 MHz): d = 8.18 (d, J = 15.2 Hz, 1H, CH@), 7.86 (d, J = 8.8 Hz,
2H, CArAH), 7.63 (d, J = 15.2 Hz, 1H, CH@), 6.78 (d, J = 9.2 Hz, 2H,
C
ArAH), 4.81 (s, 3H, N+ACH3), 3.10 (s, 6H, NA(CH3)2), 3.03 (s, 3H,
CH3), 2.99 (s, 3H, CH3). 13C NMR ((CD3)2SO, 150.8 MHz):
d = 171.2, 169.9, 168.3, 153.4, 150.2, 142.2, 138.0, 133.3, 132.8
(2 ꢀ C), 129.3, 125.7, 121.4, 118.5, 111.8 (2 ꢀ C), 105.6, 39.6
(2 ꢀ C), 38.6, 20.0, 19.9. 15N NMR ((CD3)2SO, 600 MHz): d = ꢂ79.3,
ꢂ82.3, ꢂ206.2, ꢂ309.4.
2.2.4. 2-{(E, E)-4-[4-(dimethylamino)phenyl]buta-1,3-dien-1-yl}-
3,5,8-trimethylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazolium iodide
(3b)
mp 275–277 °C. Anal.: Calcd. for C24H23IN4S3ꢁ ꢁ ꢁ2/3 CH3OH: C,
48.41; H, 4.23; N, 9.16; S, 15.72. Found: C, 47.95; H, 4.33; N,
9.12; S, 15.86. IR (powder film):
m = 3445 (AOH in CH3OH, H-
bonded), 3049 (@CAH, CArAH), 2961 and 2921 and 2851 (Csp3AH),
1615 and 1538 (C@C, CAr@CAr), 1471, 1416, 1366, 1261, 1103, 920,
821 cmꢂ1 1H NMR ((CD3)2SO, 600 MHz): d = 8.13 (dd, J = 10.9 Hz,
.
J = 14.4 Hz, 1H, CH@), 7.52 (d, J = 8.9 Hz, 2H, CArAH), 7.41 (d,
J = 15.0 Hz, 1H, CH@), 7.37 (d, J = 14.5 Hz, 1H, CH@), 7.24 (dd,
J = 11.0 Hz, J = 14.9 Hz, 1H, CH@), 6.76 (d, J = 9.0 Hz, 2H, CArAH),
4.77 (s, 3H, N+ACH3), 3.04 (s, 6H, NA(CH3)2), 3.03 (s, 3H, CH3),
3.00 (s, 3H, CH3). 13C NMR ((CD3)2SO, 150.8 MHz): d = 171.3,
168.9, 168.5, 152.1, 151.4, 148.3, 142.2, 138.1, 133.5, 130.5
(2 ꢀ C), 129.6, 126.1, 122.7, 122.2, 119.0, 112.0 (2 ꢀ C), 111.7,
39.6 (2 ꢀ C), 38.5, 20.0, 19.9. 15N NMR ((CD3)2SO, 600 MHz):
d = ꢂ79.5, ꢂ82.5, ꢂ204.2, ꢂ317.3.
2.2. Synthesis
2.2.1. 2,3,5,8-tetramethylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazo
lium iodide (2b)
2.2.5. 2-{(E, E, E)-6-[4-(dimethylamino)phenyl]hexa-1,3,5-trien-1-yl}-
trimethylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazolium iodide (3c)
mp (the dark-green color changes to red by 212 °C) decomp. at
225 °C. Anal.: Calcd. for C26H25IN4S3ꢁ ꢁ ꢁ3 CH3OH: C, 48.87; H, 5.23;
N, 7.86; S, 13.50. Found: C, 48.17; H, 4.57; N, 7.43; S, 13.13. IR
In a glass vessel for microwave reactor the compound 1 (0.3 g,
0.001 mol) was partly dissolved in methanol (20 mL). Methyl io-
dide (3.1 mL, 0.05 mol, 50 eq.) was added and the reaction mixture
was exposed to microwave irradiation for 2 ꢀ 20 min at 115 °C.
The brownish precipitate (mixture of 2a and 2b in a ratio of
15:85) was filtered off. The crude product (0.250 g) was crystal-
lized twice from methanol to get green crystals of 2b.
Yield: 0.067 g (15%); mp 230–233 °C. Anal.: Calcd for
Table 1
Synthesis of final compounds.
Entry Final
compound
Temp.
(°C)
Time
(min)
Pressure
(MPa)
Yield
(%)
C
13H12IN3S3ꢁ ꢁ ꢁ1/2 CH3OH: C, 36.08; H, 3.14; N, 9.35; S, 21.41. Found:
C, 35.64; H, 2.98; N, 9.04; S, 21.39. IR (powder film): = 3510 and
m
1
2
3
4
5
6
3a
110
115
115
120
120
120
30
15
40
90
70
70
0.60
0.60
0.62
0.64
0.67
0.70
78
77
28
62
45
87
3435 (AOH in CH3OH, H-bonded), 3047, 2974 and 2929 and 2900
3b
3c
3d
3e
3f
and 2859 (Csp3AH), 2721, 2379, 1983, 1603 and 1578 (CAr@CAr),
1506, 1429, 1365, 1163 cmꢂ1 1H NMR ((CD3)2SO, 300 MHz):
.
d = 4.80 (s, 3H, N+ACH3), 3.29 (s, 3H, CH3), 3.16 (d, J = 4.3 Hz, CH3
from CH3OH), 3.05 (s, 3H, CH3), 3.010 (s, 3H, CH3). 13C NMR

72
A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
(powder film):
m
= 3441 (AOH in CH3OH, H-bonded), 3068 and
N+ACH3), 3.37 (t, J = 5.5 Hz, 4H, CH2), 3.00 (s, 3H, CH3), 2.97 (s,
3H, CH3), 2.70 (t, J = 6.0 Hz, 4H, CH2), 1.91–1.87 (m, 4H, CH2). 13C
NMR ((CD3)2SO, 150.8 MHz): d = 171.6, 169.4, 168.6, 150.6, 148.5,
142.8, 138.3, 133.8, 131.1 (2 ꢀ C, very weak signal), 129.6, 125.7,
121.7, 121.3 (2 ꢀ C), 118.4, 104.1, 50.2 (2 ꢀ C), 38.7, 27.3 (2 ꢀ C),
21.0 (2 ꢀ C), 20.6, 20.4. 15N NMR (DMSO, 600 MHz): d = ꢂ79.3,
ꢂ82.3, ꢂ211.4, ꢂ287.0.
3035 (@CAH, CArAH), 2957 and 2908 and 2851 (Csp3AH), 2794,
2726, 2653, 1612 and 1574 (C@C, CAr@CAr), 1509, 1470, 1385,
1236, 1203, 1097, 998, 815 cmꢂ1 1H NMR ((CD3)2SO, 600 MHz):
.
d = 8.08 (dd, J = 11.1 Hz, J = 14.4 Hz, 1H, CH@), 7.45 (d, J = 8.9 Hz,
2H, CArAH), 7.42 (d, J = 14.6 Hz, 1H, CH@), 7.32–7.27 (m, 1H,
CH@), 7.03–7.02 (m, 2H, CH@), 6.81 (dd, J = 11.3 Hz, J = 14.1 Hz,
1H, CH@), 6.68 (d, J = 9.0 Hz, 2H, CArAH), 4.78 (s, 3H, N+ACH3),
3.03 (s, 3H, CH3), 3.00 (s, 3H, CH3), 2.99 (s, 6H, NA(CH3)2). 13C
NMR ((CD3)2SO, 150.8 MHz): d = 171.9, 169.5, 169.1, 151.7, 150.9,
149.9, 143.7, 142.7, 138.7, 134.1, 130.2, 130.0 (2 ꢀ C), 128.9,
126.8, 124.1, 124.0, 119.9, 113.5, 112.3 (2 ꢀ C), 40.2 (2 ꢀ C), 39.3,
20.6, 20.5. 15N NMR ((CD3)2SO, 600 MHz): d = ꢂ79.5, ꢂ82.4,
ꢂ202.7, ꢂ322.4.
2.2.9. 2-[(E)-2-(9-julolidinyl)vinyl]-3-methylbenzothiazolium iodide
(7f)
To the solution of 2,3-dimethylbenzothiazolium iodide (0.87 g,
0.003 mol) in methanol (15 mL), 9-julolidinyl)carbaldehyde
(0.6 g, 0.003 mol) and a few drops of pyridine were added. The
reaction mixture was heated to reflux for 12 h. The dark precipitate
with metallic lustre was filtered off and washed with diethyl ether.
2.2.6. 2-{(E)-2-[4-(diphenylamino)-phenyl]vinyl}-3,5,8-trimeth
ylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazolium iodide (3d)
mp 187–190 °C. Anal.: Calcd. for C32H25IN4S3ꢁ ꢁ ꢁ2/3 CH3OH: C,
55.26; H, 3.93; N, 7.89; S, 13.55. Found: C, 55.09; H, 3.74; N,
Yield: 1.2 g (85%); mp 233–234 °C. IR (powder film):
m = 3425
(AOH in CH3OH, H-bonded), 3058 and 3012 (@CAH, CArAH),
2933 and 2841 and 2706 and 2609 (Csp3AH), 1617 and 1569
(C@C, CAr@CAr), 1518, 1494, 1453, 1315, 1293, 1157 cmꢂ1 1H
.
7.88; S, 14.01. IR (powder film):
m = 3472 (AOH in CH3OH, H-
NMR ((CD3)2SO, 600 MHz): d = 8.23 (d, J = 7.8 Hz, 1H, CArAH),
8.01 (d, J = 7.8 Hz, 1H, CArAH), 7.88 (d, J = 15.0 Hz, 1H, CH@), 7.74
(dt, J = 7.8 Hz, J = 0.9 Hz, 1H, CArAH), 7.63 (dt, J = 7.8 Hz, J = 0.9 Hz,
1H, CArAH), 7.44 (d, J = 15.0 Hz, 1H, CH@), 4.16 (s, 3H, N+ACH3),
3.37 (t, J = 5.9 Hz, 4H, CH2), 2.73 (t, J = 5.8 Hz, 4H, CH2), 1.92–1.88
(m, 4H, CH2). 13C NMR ((CD3)2SO, 150.8 MHz): d = 170.8, 150.6,
148.3, 142.4, 130.8 (2 ꢀ C, weak signal) 129.1, 127.4, 126.9,
124.1, 121.6, 121.0 (2 ꢀ C), 115.9, 104.6, 50.1 (2 ꢀ C), 35.6, 27.4
(2 ꢀ C), 21.1 (2 ꢀ C). 15N NMR ((CD3)2SO, 600 MHz): d = ꢂ212.2,
ꢂ289.4.
bonded), 3054 and 3032 and 3011 (@CAH and CArAH), 2990 and
2965 and 2820 and 2904 (Csp3AH), 2850, 2751, 2671, 2505, 1569
and 1545 and 1504 and 1485 (C@C, CAr@CAr), 1435, 1367, 1291,
1258, 1173, 1034, 946, 819, 700 cmꢂ1
.
1H NMR ((CD3)2SO,
600 MHz): d = 8.29 (d, J = 15.5 Hz, 1H, CH@), 7.92 (d, J = 8.9 Hz,
2H, CArAH), 7.82 (d, J = 15.5 Hz, 1H, CH@), 7.45 (t, J = 8.1 Hz, 4H,
C
C
ArAH), 7.27 (t, J = 7.4 Hz, 2H, CArAH), 7.22 (d, J = 7.5 Hz, 4H,
ArAH), 6.92 (d, J = 8.8 Hz, 2H, CArAH), 4.87 (s, 3H, N+ACH3), 3.03
(s, 3H, CH3), 2.99 (s, 3H, CH3). 13C NMR ((CD3)2SO, 150.8 MHz):
d = 172.0, 170.8, 169.2, 152.0, 149.5, 145.8 (2 ꢀ C), 142.7, 138.8,
134.1, 132.4 (2 ꢀ C), 130.5 (4 ꢀ C), 130.2, 127.0, 126.7 (4 ꢀ C),
126.6, 126.1 (2 ꢀ C), 120.0, 119.2 (2 ꢀ C), 110.1, 39.7, 20.6, 20.5.
15N NMR ((CD3)2SO, 600 MHz): d = ꢂ79.6, ꢂ82.4, ꢂ199.9, ꢂ269.7.
2.2.10. Quaternization with dimethyl sulfate
Compound 1 (0.03 g, 0.1 mmol) was partly dissolved in toluene
(3 mL). Dimethyl sulfate (0.29 mL, 0.003 mol, 30 eq.) was added
and the reaction mixture was heated at 110 °C for 8 h. Toluene
was evaporated under vacuum and the crude product was washed
several times with diethyl ether. The composition of the product
was established by 1H NMR and is discussed below (Sections 3.1
and 3.3).
2.2.7. 2-{(E, E)-4-[4-(diphenylamino)phenyl]buta-1,3-dien-1-yl}-
3,5,8-trimethylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazolium iodide
(3e)
mp 178–181 °C. Anal.: Calcd. for C34H27IN4S3ꢁ ꢁ ꢁ2/3 CH3OH: C,
56.57; H, 4.06; N, 7.61; S, 13.07. Found: C, 56.21; H, 3.78; N,
7.43; S, 13.24. IR (powder film):
m = 3479 and 3407 (AOH in
CH3OH, H-bonded), 3054 and 3031 (@CAH and CArAH), 2966 and
2920 (Csp3AH), 2692, 2632, 2555, 2309, 2004, 1540 and 1506
and 1475 (C@C, CAr@CAr), 1406, 1331, 1300, 1262, 1150, 1099,
2.2.11. Quaternization with trimethyloxonium tetrafluoroborate
In a glass vessel for microwave reactor the compound 1 (0.1 g,
0.34 mmol) was dissolved in dried 1,2-dichloroethane (5 mL).
Trimethyloxonium tetrafluoroborate (0.251 g, 1.7 mmol, 5 eq.)
was added and the reaction mixture, under inert atmosphere,
was exposed to microwave irradiation for 5 ꢀ 10 min at 165 °C.
The brownish precipitate was filtered off and the composition of
the crude product (0.156 g) was established by 1H NMR and is dis-
cussed below (Sections 3.1 and 3.3).
999, 945, 831, 698 cmꢂ1 1H NMR ((CD3)2SO, 600 MHz): d = 8.17
.
(dd, J = 10.6 Hz, J = 14.6 Hz, 1H, CH@), 7.56 (d, J = 8.8 Hz, 2H,
ArAH), 7.55 (d, J = 14.7 Hz, 1H, CH@), 7.42–7.40 (m, 4H, CArAH),
C
7.40 (d, J = 15.9 Hz, 1H, CH@), 7.31 (dd, J = 10.5 Hz, J = 15.2 Hz,
1H, CH@), 7.21–7.18 (m, 2H, CArAH), 7.16–7.14 (m, 4H, CArAH),
6.92 (d, J = 8.8 Hz, 2H, CArAH), 4.81 (s, 3H, N+ACH3), 3.03 (s, 3H,
CH3), 3.00 (s, 3H, CH3). 13C NMR ((CD3)2SO, 150.8 MHz):
d = 172.1, 169.8, 169.3, 150.9, 150.2, 146.5, 146.4 (2 ꢀ C), 142.7,
138.8, 134.1, 130.38, 130.35 (4 ꢀ C), 130.3 (2 ꢀ C), 128.4, 127.1,
126.1 (4 ꢀ C), 125.4, 125.3 (2 ꢀ C), 120.8 (2 ꢀ C), 120.2, 114.9,
39.5, 20.6, 20.5. 15N NMR ((CD3)2SO, 600 MHz): d = ꢂ79.6, ꢂ82.5,
ꢂ200.0, ꢂ273.7.
2.2.12. Quaternization with 1,3-propanesultone
Compound 1 (0.05 g, 0.17 mmol) was dissolved in toluene
(2 mL) and 1,3-propanesultone (0.060 mL, 0.68 mmol, 4 eq.) was
added. The reaction mixture was heated at 110 °C for 19 h. The
white precipitate was filtered off and washed with diethyl ether.
The composition of the crude product (0.055 g) was established
by 1H NMR.
2.2.8. 2-[(E)-2-(9-julolidinyl)vinyl]-3,5,8-trimethylbisthiazolo[4,5-e;
5,4-g]-1,3-benzothiazolium iodide (3f)
mp 225–228 °C. Anal.: Calcd. for C26H25IN4S3 ꢁ 1/2 CH3OH: C,
50.31; H, 4.30; N, 8.86; S, 15.21. Found: C, 50.04; H, 4.05;
2.2.13. 1H NMR for 3-(2,5,8-trimethylbisthiazolo[4,5-e; 4,5-g]-1,3-
benzothiazolium-3-yl)propane-1-sulfonate (6a) and for 3-(2,5,8-
trimethylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazolium-3-yl)propane-
1-sulfonate (6b)
Minor isomer: (CD3OD, 300 MHz): d = 5.61–5.57 (m, 2H,
N+ACH2A), 3.35 (s, 3H, CH3), 3.12 (t, J = 7.0 Hz, 2H, CH2), 3.062 (s,
3H, CH3), 3.002 (s, 3H, CH3), 2.56–2.46 (m, 2H, CH2).
N, 8.90; S, 15.71. IR (powder film):
m = 3444 (AOH in CH3OH, H-
bonded), 3044 and 3001 (@CAH and CArAH), 2923 and 2837
(Csp3AH), 2710, 1621 and 1566 and 1520 (C@C, CAr@CAr), 1481,
1433, 1410, 1315, 1232, 1208, 1147, 1104, 907, 822 cmꢂ1
.
1H
NMR ((CD3)2SO, 600 MHz): d = 7.96 (d, J = 14.9 Hz, 1H, CH@), 7.47
(s, 2H, ArAH), 7.41 (d, J = 14.9 Hz, 1H, CH@), 4.71 (s, 3H,
C

A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
73
Table 2
1.41(1) Å to obtain reasonable geometry. Thermal parameters of
the H atoms were set to Uiso(H) = 1.5 Ueq(C, O).
Calculated quantum chemical descriptors related to nonlinear optical properties of
studied benzothiazolium cations: energy difference gap between HOMO and LUMO
orbitals (
D
E), bond length alternation index (BLA), polarizability
a
, first and second
hyperpolarizabilities b and
c
.
2.4. Computational details
Compound
D
(eV)
E
BLA
(Å)
a
b
c
(ꢀ10ꢂ24 esu) (ꢀ10ꢂ30 esu) (ꢀ10ꢂ36 esu)
The optimal geometries for each compound in vacuo were cal-
culated in the Turbomole 6.2 program [25], using the DFT method
[26] and the B3LYP exchange–correlation functional [27]. On all
atoms the TZVP basis set [28] was employed. For calculating the
ESP charges a parametrization very close to that by Singh and
Kollman [29] was used. The time dependent DFT method [30]
was used to calculate the first excited state.
3a
3b
3c
3d
3e
3f
2.64
2.36
2.06
2.41
2.13
2.53
0.043
138
230
427
667
341
657
236
217
119
1181
667
564
91
0.0325 183
0.027 240
0.0425 184
0.032
0.036
233
153
First order hyperpolarizabilities (b) [31] were calculated using
the method PM3 [32] from the program package Ampac 8 [33].
Major isomer: (CD3OD, 300 MHz): d = 5.59–5.53 (m, 2H,
N+ACH2A), 3.36 (s, 3H, CH3), 3.12 (t, J = 7.0 Hz, 2H, CH2), 3.055 (s,
3H, CH3), 2.995 (s, 3H, CH3), 2.56–2.46 (m, 2H, CH2).
3. Results and discussion
2.3. X-ray data collection, structure solution and refinement
3.1. Synthesis and calculations
The green crystal of the compound 2b was mounted on a cactus
thorn by epoxy glue. Measurement was taken on an Oxford Diffrac-
The six step synthesis of the unsymmetrical trimethylbenzotris-
thiazole (BTT) we reported recently [14] (Scheme 4).
tion Gemini R
chromated Mo K
(293 K) using
absorption effects were applied using the CrysAlisPro software
[20]. The structure was solved by structure-invariant direct
methods using VLD procedure in SIR-2011 [21] and refined using
SHELXL-97 [22]. Molecular graphic employed include DIAMOND
[23] and Mercury [24]. The crystal data and details of data collec-
tion and refinement are given in Table 3.
j
-axis diffractometer [20] with a graphite-mono-
radiation (k = 0.71073 Å) at room temperature
scans. Corrections for Lorentz, polarization and
To enhance the electron withdrawing character of the
benzotristhiazole building block we have tried to methylate
the heterocyclic nitrogens. Firstly we have performed the quantum
chemical calculations of the starting trimethylbenzotristhiazole (1)
and monomethylated products by DFT methods. The atom cen-
tered ESP charges on nitrogen atoms were used as a measure of
nucleophilicity (Fig. 1). According to the calculations the methyla-
tion of the nitrogen in b-ring (N-b) is preferred to N-a. The least
charge density was calculated on N-c.
a
x
All non-H atoms were refined anisotropically as free atoms. The
hydrogen atoms were refined by using the riding model with SHEL-
XL-default d(CAH) values. C10, C13 and C20 methyl groups were
refined with staggered geometry, C11 and C12 methyl groups were
refined as disordered (freely rotating). H1 atom was refined as
DFT total energy values of methylated cations can serve for
comparison of the thermodynamic stability of possible monome-
thylated products. Values of relative energies are presented in
Fig. 2. The cation methylated on N-b shows the least value of en-
ergy, the product of methylation on N-a is slightly higher in energy
and the third isomer methylated on N-c is least probable to be
formed. As a conclusion of quantum chemical calculations we
follows:
y coordinate was fixed to 0.25 and bond lengths
d(H1AO1) and d(H1AC20) were restrained to 0.82(1) Å and
Table 3
Experimental data for 2b.
Empirical formula
C14H16 I N3OS3
465.38
Mr
Crystal system, space group
Unit cell dimensions
orthorhombic, Pnma
Determined from 7684 reflections
24.1828 (6), 90
6.8113 (1), 90
10.7365 (3), 90
1768.47 (7)
a (Å),
b (Å), b (°)
c (Å), (°)
V (Å3)
a
(°)
c
Z, density (calcd.) (g cmꢂ3
)
4, 1.748
l
2.159 mmꢂ1
F(000)
920
Absorption correction, Tmin, Tmax
Crystal size (mm)
Crystal habit, color
Analytical, 0.551, 0.916
0.88 ꢀ 0.18 ꢀ 0.04
Plate, green
h Range for data collection (°)
Limiting indices
3.5–29.5
h = ꢂ31 ? 32
k = ꢂ9 ? 8
l = ꢂ14 ? 13
Reflections collected/unique/observed[I > 2
Completeness to h (°)
Data/restraints/parameters
r(I)]
28599/2526/1916 [Rint = 0.028]
99.0% to 28.5°
2526/2/135
2
w ¼ 1=½r2ðFo2Þ þ ð0:0397PÞ þ 1:1107Pꢃ where P ¼ ðFo2 þ 2F2c Þ=3
Final weighting scheme
S/restrained S
1.084/1.084
Final R indices [I > 2
r
(I)]
R = 3.63%, wR = 8.10%
R = 5.57%, wR = 8.88%
0.64/ꢂ0.91
Final R indices (all data)
Largest difference peak/hole (e Åꢂ3
)

74
A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
Fig. 3. MEP visualization of the computed nitrogen electron densities for cat_a and
cat_b.
Fig. 1. (a) Calculated ESP charges (Q ꢀ 103 values are presented) on nitrogens of
starting neutral compound BTT. (b) MEP visualization of the computed nitrogen
electron densities.
(2) Methylation with dimethyl sulfate (Scheme 2, option A)
offered a mixture of mono- (13%) and dimethylated salts
(78%). The two monomethyl isomers (4a and 4b) were
formed in a ratio of 2:3 and the two dimethyl isomers (4c
and 4d) were formed in a ratio of 1.15:1. A small amount
(9%) of the starting compound was also observed.
can predict the probability of formation of monomethylated prod-
ucts: cat_b > cat_a ꢄ cat_c.
The structure of dimethylated salts can be also predicted based
on calculated total energy values of isomers. The results shown in
Fig. 2 represent relative energy values of the three conceivable
dications. It is possible to exclude the isomer dicat_ab due to the
repulsion of neighboring methyl groups and repulsion of quite near
positive charges on nitrogens N-a and N-b. The isomers dicat_ac
and dicat_bc are much more likely to be formed as could be also
seen from the MEP visualization of the computed nitrogen electron
densities (Fig. 3). The highest electron density is on the N-c.
The quaternization reactions were realized with several alkylat-
ing agents:
(3) Trimethyloxonium tetrafluoroborate is known as a powerful
alkylating agent (Scheme 2, option B). With the 5-fold excess
of (CH3)3 O+ BFꢂ we have obtained a mixture of monomethy-
4
lated isomers (53%), dimethylated isomers (37%) and 10% of
untreated starting BTT. The two monomethyl isomers (5a
and 5b) as well as the two dimethyl isomers (5c and 5d)
have been formed in a ratio of 1:1. A further quaternization
of the crude product with another 3-fold excess of (CH3)3 O+
BFꢂ4 have not changed the composition of the mixture.
(4) A mixture of two monoalkylated products in a ratio of 65:35
has been obtained using excess of 1,3-propanesultone as an
alkylating reagent (Scheme 3). Which of the two isomers is
the major one, we deduced from the calculated total ener-
gies and by comparing the 1H NMR spectrum with those of
cat_a and cat_b. According to the relative total energies
given in Fig. 4 the isomer 6b is slightly preferred. Comparing
the 1H NMR shifts for the methyl group bound to the carbon
in the alkylated ring with the corresponding one in cat_a
resp. cat_b, we can see that the one for cat_a (d = 3.269)
and the minor isomer (d = 3.35) are lower shifted than that
for cat_b (d = 3.292) and the major isomer (d = 3.36). Similar
shifts can be observed for the CH2 groups bound directly to
the nitrogen and the corresponding methyl groups in cat-
ions. The signals for cat_a (d = 4.808) and the minor isomer
(d = 5.61–5.57) are higher shifted than that for cat_b
(d = 4.803) and the major isomer (d = 5.59–5.53). So we can
conclude that the major isomer could be compound 6b.
(1) Alkylation of BTT with an excess of methyl iodide in metha-
nol ran with a high conversion and produced a mixture of
two monomethylated salts. The solid product which precip-
itated from the reaction mixture consisted of the isomeric
salts 2a and 2b (Scheme 1) in a ratio of 3:7. The opposite
ratio of monomethylated products (7:3) was determined in
the filtrate. To summarize the results we have taken into
account the amount of the precipitate (59%) and that of
the filtrate (41%). The final ratio of monomethylated prod-
ucts was 46:54, roughly 1:1. The precipitate was further
treated and the prevailing isomer (2b) was separated by
crystallization. Its right structure was proved by X-ray anal-
ysis. A further quaternization either of the mixtures 2a and
2b or of the pure isomer 2b with dimethyl sulfate or with
trimethyloxonium tetrafluoroborate gave the dimethylated
products only in a yield of 10%.
Fig. 2. Calculated relative energies of the three possible monomethylated cations (cat_a, cat_b, cat_c) and three possible dimethylated cations (dicat_ab, dicat_ac, dicat_bc).

A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
75
Scheme 1. The synthesis of final products.
Scheme 2. Methylation of 1 with (A) dimethyl sulfate (products 4a–4d); (B) trimethyloxonium tetrafluoroborate (products 5a–5d).
We performed the reaction with 1,3-propanesultone to obtain a
salt with a better solubility. The mixture of isomers 6a and 6b is
very well soluble in methanol, but insoluble in DMSO whereas
the previous methylated compounds dissolve reversely.
The last step in the synthesis of the final compounds 3a–3f is
the Knoevenagel reaction of 2b with aromatic aldehydes contain-
ing an electron-releasing group (Scheme 1). These reactions are
performed under microwave irradiation which shortens the reac-
tion time and the obtained yields are very good (Table 1). Similarly
to the compound 2b (that was confirmed by X-ray analysis) we
presume that compounds 3a–3f contain in their crystalline struc-
ture molecules of methanol. It can be seen in the 1H NMR spectra
and IR spectra (strong signals for AOH hydrogen bonds). Even dry-
ing the compounds under high vacuum for 8 h does not remove the
methanol from the crystalline structure.
Calculated quantum-chemical descriptors related to the nonlin-
ear properties of studied cations are reported in Table 2. Small
HOMO–LUMO energy gap (DE) predicts interesting first hyperpo-
larizability b as can be seen in compounds 3c and 3e. The BLA index
represents the measure of conjugation and the smallest values are

76
A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
Scheme 3. Reaction with 1,3-propanesultone.
Further properties of compounds 3a–3f are discussed in Section
3.4 of this article.
3.2. X-ray crystal structure of compound 2b
The crystal of the compound 2b was obtained by slow evapora-
tion of methanol solution at room temperature. Title compound
crystallizes as crystallosolvate with methanol (molar ratio 1:1) in
orthorhombic Pnma space group with unit cell parameters
a = 24.1828 (6) Å, b = 6.8113 (1) Å, c = 10.7365 (3) Å, V = 1768.47
(7) Å3 and Z = 4. The crystal structure of 2b is given in Fig. 5 and
packing pattern is given in Fig. 6. Selected geometry features are
given in Table 5.
Scheme 4. Trimethylbenzotristhiazole.
again observed in cations 3c and 3e. These cations are also charac-
terized by the highest values of linear polarizability
important parameters are hyperpolarizabilities b and
a
. The most
2,3,5,8-tetramethylbisthiazolo[4,5-e; 5,4-g]-1,3-benzothiazoli-
um cation, iodide anion and methanol molecule are located on mir-
ror planes of unit cell. Cation contains weak intramolecular
C13AH13Aꢁ ꢁ ꢁN2 bond (Table 4). Intermolecular interactions con-
c, which pre-
dict the structures with possible application in nonlinear optics. As
can be seen from Table 2 hyperpolarizability b is influenced by the
number of double bonds in the conjugated bridge. The exchange of
the electrondonor NMe2 for NPh2 group causes a significant in-
crease in the b values (3a ? 3d, 3b ? 3e). The b values for 3c
and 3e seem to be quite promising. Even if a direct relation be-
sist of
p–p stacking interactions between cations lying in neigh-
boring mirror planes and of hydrogen bonding network
containing both intraplanar and interplanar interactions. Intrapla-
nar are stronger of them: d(Hꢁ ꢁ ꢁA) is more than 0.5 Å shorter than
sum of VdW radii for O1AH1ꢁ ꢁ ꢁI1 and more than 0.2 Å shorter for
C10AH10Aꢁ ꢁ ꢁI1 interaction. Interplanar interactions are based on
crystallographically equivalent C10AH10B/Cꢁ ꢁ ꢁI1, which are some-
what weaker than interplanar CAHꢁ ꢁ ꢁI interaction. (All d(Hꢁ ꢁ ꢁA)
distances discussed here are based of constrained H-atoms posi-
tions for X-ray data, real distances are even shorter [34].)
As can be seen from the Table 5, the calculated values corre-
spond very well with the experimental. The differences in CAC
bonds are only 0.002–0.016 Å and in CAN bonds only
0.002–0.008 Å. Exception is the bond C13AN1, where the differ-
tween second hyperpolarizability
cations is not so evident, the unexpected high value of
c
and structural units of studied
for 3c was
c
surprising and encouraging. Based on these findings we have com-
bined the best structural units: three double bonds and diphenyl-
amino donor substituent to design and calculate the new predicted
cation. Its structure can be deduced from Scheme 1: compound 3,
with n = 3, R = NPh2. The calculations fulfilled our expectation: the
predicted structure has a minimal HOMO–LUMO energy gap
(D
E = 1.91 eV), polarizability
a
= 294 ꢀ 10ꢂ24 esu and excellent
first hyperpolarizability (b = 1012 ꢀ 10ꢂ30 esu).
Fig. 4. The relative total energies for the isomers 6a, 6b and 6c.

A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
77
Table 4
Hydrogen bonding interactions in the structure of 2b (Å, °).
DAHꢁ ꢁ ꢁA
DAH
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
DAHꢁ ꢁ ꢁA
Intramolecular
C13AH13Aꢁ ꢁ ꢁN2
0.96
2.21
3.001 (5)
139
Intermolecular–intraplanar
O1AH1ꢁ ꢁ ꢁI1
0.82 (1)
0.96
2.67 (1)
2.97
3.488 (4)
3.932 (4)
176 (2)
178
C10AH10Aꢁ ꢁ ꢁI1a
Intermolecular–interplanar
C10AH10Bꢁ ꢁ ꢁI1
0.96
0.96
3.05
3.05
4.001 (2)
4.001 (2)
173
173
C10AH10Cꢁ ꢁ ꢁI1b
Symmetry codes:
a
ꢂx + 1/2, ꢂy + 1, z + ½.
b
x, y + 1, z.
15N NMR spectra of compounds 3a–3f show that the signals for
neutral nitrogens from the benzotristhiazolium core are always
shifted around ꢂ80 ppm. Signals for the quaternized nitrogens
can be found from ꢂ199.9 ppm to ꢂ211.4 ppm. The shifts for the
nitrogens from ANMe2 and ANPh2 groups or julolidine moiety vary
from ꢂ269.7 ppm to ꢂ322.4 ppm.
Fig. 5. Crystal structure of compound 2b with numbering scheme. All non-
hydrogen atoms are drawn as 50% probability ellipsoids; H atoms are drawn as
spheres of arbitrary small radius for clarity.
ence is 0.038 Å. The largest differences are in CAS bonds (0.014–
0.053 Å). Comparing the values for the angles, we can see that
the differences are in the range 0–2.5°. The computed dihedral an-
gles show, that the structure is planar which was also confirmed by
the experiment.
3.4. UV–Vis and emission spectroscopy
The UV–Vis absorption and emission characteristics of the tar-
get salts 3a–3f are presented in Table 6. The compounds under
study absorb in the visible region and are characterized by a strong
long wave ICT band with molar absorption coefficient of about
50,000–100,000 Mꢂ1 cmꢂ1. The high value of the molar absorption
3.3. NMR spectroscopy
coefficient indicates an extensive conjugation of p-electrons sug-
gesting a planar structure of the molecule in its ground state.
The results can be compared to those of the corresponding one
dimensional benzothiazolium salts 7a–7f (Fig. 7) with push–pull
structure, which have been published by our group [18,35].
The two thiazole cycles in salts 3a–3f could be understood as
auxiliary units enhancing the electron withdrawing capacity of
the benzothiazolium moiety. This structural motif results in the
red shift of the long wave absorption band as well as in higher val-
The prepared monomethylated and dimethylated salts except
the compound 2b, which structure was fully determined by
X-ray, were identified by 1H NMR spectroscopy. The values for each
cation are given in the following. The shifts are not dependent on
the counterion. To determine which signals belong to dicat_ac
and which to dicat_bc we performed a reaction, where we further
quaternized compound 2b with dimethyl sulfate. According to 1H
NMR a yield of approximately 30% was estimated, but we could
precisely assign the signals belonging to dicat_bc.
ues of extinction coefficients
e compared to the corresponding lin-
ear structures. The comparison of kA values for salts 7a–7f and the
corresponding neutral benzothiazoles [35] indicates that the qua-
ternary nitrogen in benzothiazolium core shows a serious influ-
ence on the high withdrawing character of the benzothiazolium
unit (high bathochromic shift) compared to the corresponding
neutral benzothiazoles.
The expected bathochromic shift was also observed by prolon-
gation of the p-bridge connecting the donor and the acceptor parts
of studied compounds. The exchange of NMe2 substituent for NPh2
caused a hypsochromic shift of long wave band as well as reduc-
tion of extinction coefficients, probably owing to lower planarity
of the system [18]. On the contrary, the julolidine donor substitu-
cat_a 1H NMR ((CD3)2SO, 300 MHz): d = 4.808 (s, 3H, N+ACH3),
3.269 (s, 3H, CH3), 3.060 (s, 3H, CH3), 3.006 (s, 3H, CH3).
cat_b 1H NMR ((CD3)2SO, 300 MHz): d = 4.803 (s, 3H, N+ACH3),
3.292 (s, 3H, CH3), 3.054 (s, 3H, CH3), 3.010 (s, 3H, CH3).
dicat_ac 1H NMR ((CD3)2SO, 300 MHz): d = 4.820 (s, 3H,
N+ACH3), 4.502 (s, 3H, N+ACH3), 3.341 (s, 3H, CH3), 3.325 (s,
3H, CH3), 3.155 (s, 3H, CH3).
dicat_bc 1H NMR ((CD3)2SO, 300 MHz): d = 4.872 (s, 3H,
N+ACH3), 4.585 (s, 3H, N+ACH3), 3.378 (s, 3H, CH3), 3.357 (s,
3H, CH3), 3.125 (s, 3H, CH3).
Fig. 6. View of the cell packing along the c axis. Dashed lines indicate the hydrogen bonding interactions.

78
A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
Table 5
Selected geometry features of compound 2b.
Parameters
Calculated
Experimental
Parameters
Calculated
Experimental
Bond lengths (Å)
Angles (°)
C1AC3
C1AC7
C1AS1
C2AN1
C2AC10
C2AS1
C3AC4
C3AN1
C4AN2
C4AC5
C5AC6
C5AS2
C6AC7
C6AS3
C7AN3
C8AN2
C8AC11
C8AS2
C9AN3
C9AC12
C9AS3
C13AN1
1.395
1.402
1.748
1.330
1.487
1.719
1.418
1.404
1.376
1.409
1.401
1.747
1.404
1.747
1.372
1.294
1.490
1.780
1.292
1.489
1.792
1.480
1.382 (5)
1.417 (5)
1.726 (4)
1.322 (5)
1.474 (5)
1.705 (4)
1.409 (5)
1.410 (4)
1.372 (5)
1.407 (5)
1.408 (5)
1.724 (4)
1.388 (5)
1.730 (3)
1.382 (5)
1.292 (5)
1.497 (6)
1.741 (4)
1.296 (5)
1.498 (6)
1.739 (5)
1.442 (5)
C3AC1AC7
C1AC3AC4
C5AC4AC3
C4AC5AC6
C7AC6AC5
C6AC7AC1
N1AC2AS1
C10AC2AS1
N1AC2AC10
N2AC8AS2
C11AC8AS2
N2AC8AC11
N3AC9AS3
C12AC9AS3
N3AC9AC12
C2AN1AC3
C2AN1AC13
C3AN1AC13
C8AN2AC4
C9AN3AC7
C2AS1AC1
C5AS2AC8
C6AS3AC9
122.1
119.7
118.1
121.5
120.2
118.4
113.1
122.8
124.2
114.6
120.5
124.9
114.7
120.3
125.1
114.3
121.5
124.2
112.2
111.6
90.33
88.85
88.63
120.8 (3)
121.1 (3)
118.1 (3)
120.7 (3)
120.8 (3)
118.5 (3)
113.3 (3)
122.5 (3)
124.2 (3)
115.7 (3)
120.4 (3)
123.9 (4)
116.4 (3)
120.2 (4)
123.4 (4)
113.8 (3)
121.9 (3)
124.2 (3)
110.9 (3)
109.1 (3)
90.35 (18)
89.28 (18)
89.26 (19)
Dihedral angles (°)
C10AC2AN1AC3
C10AC2AN1AC13
S1AC2AN1AC13
C1AC3AN1AC13
C10AC2AS1AC1
ꢂ179.8
1.17
180.0
0.0
180.0
180.0
180.0
C11AC8AN2AC4
C11AC8AS2AC5
C12AC9AN3AC7
C12AC9AS3AC6
179.9
180.0
180.0
180.0
180.0
180.0
ꢂ179.1
ꢂ179.9
179.2
179.7
179.9
Table 6
kA – Absorption maximum, kF – fluorescence maximum,
e
– molar absorption coefficient, mA ꢂ mF – Stokes shift, UF – fluorescence quantum yield. Used solvent: methanol.
Compound
kA (nm)
e
(Mꢂ1 mꢂ1
)
Compound
kA (nm)
e
(Mꢂ1 cmꢂ1
)
kF (nm)
mA
ꢂ
mF (nm)
UF
7a
7b
7c
7d
7e
7f
523
562
580
512
537
567
62,400
58,200
46,300
36,800
52,800
3a
3b
3c
3d
3e
3f
540
580
598
528
550
586
63,800
76,300
54,300
53,900
52,600
98,000
612
718
832
710
782
635
72
0.011
0.039
0.105
0.0023
0.0035
0.0072
138
234
182
232
49
10,1000
ent causes extraordinary red shift with the highest extinction coef-
ficient. Unlike the ANMe2 and ANPh2 donors, the nitrogen in julo-
lidine substituent is rigidized and rehybridized from pyramidal to
planar configuration. This structure enables better conjugation and
more effective charge transfer. For comparison we have also pre-
pared the corresponding linear benzothiazolium salt 7f with julo-
lidine donor, which confirmed the observed trends.
The effective charge transfer in benzothiazolium cations 3a–3f
and 7a–7f enables a good overlap between the squares of the
vibrational wave functions of the initial and final electronic states
which results in limited emission (rapid radiationless transfer). Ob-
served low fluorescence quantum yield values (UF) for 3a–3f and
7a–7f [18] confirmed this result. The UF values for 3a–3e are en-
hanced with increasing conjugation length of the chromophoric
system. We assume that this behavior is associated with the rota-
tion around the CAC single bonds in conjugated bridge (i.e. intra-
molecular twisting) [36]. Enhanced conjugation leads to an
increase in the electron density distribution of the CAC single
bonds. The higher electron density distribution of CAC single
bonds hinders the twisting of these bonds and therefore results
Fig. 7. Comparison of final structures (3a–3f) with linear compounds (7a–7f).

A. Fülöpová et al. / Journal of Molecular Structure 1027 (2012) 70–80
79
Fig. 8. Relevant molecular orbitals (isosurface 0.04 au) in the compound 3a.
in higher UF values for chromophores with longer conjugated
bridge. The Stokes shift is also enlarged with the prolongation of
conjugation. The smallest Stokes shift was observed in compound
3f with julolidine donor and could be explained by the more sim-
ilar values of the excited and ground state dipole moments. While
in compounds 3a–3e the dipole moment of the excited state differs
more from the ground state’s one, in compound 3f, owing to better
conjugation, the important charge transfer have already occurred
in the ground state. The modest difference in the charge transfer
character of excited and ground state is the reason of small Stokes
shift in 3f.
predicted and a high value of b was obtained by theoretical calcu-
lations that agree with the prediction.
Supplementary material
The deposition number CCDC 876258 for compound 2b
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.a-
c.uk/data_request/cif or Cambridge Crystallographic Data Center,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Fig. 8 presents an orbital picture of cation 3a. Highest occupied
molecular orbital (HOMO) is predominantly located on the donor
part (dimethylamino and phenyl) whereas in the lowest unoccu-
pied molecular orbital (LUMO) the orbital density is moved to
the thiazolium part of the heterocyclic core and conjugated bridge.
This corresponds with the lowest energy excitation (ICT band)
which is represented almost exclusively (more than 97%) by the
HOMO–LUMO transition. Orbitals HOMO ꢂ 1 as well as LUMO + 1
are localized in the benzotristhiazolium part of the cation.
Acknowledgements
This work has been supported by the Slovak Grant Agency APVV
(Grants APVV 0259-07, APVV 0424-10 and APVV 0262-10) and also
by the Slovak Grant Agency for Science (Grant no. 1/1126/11). The
financial support is gratefully acknowledged. A.F. thanks for the
Comenius University Grants (No. UK/56/2010, UK/259/2009). We
also thank the Structural Funds, Interreg IIIA for the financial sup-
port in purchasing the diffractometer.
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